1. Field of the invention
The invention relates to an improved electrical machine, in particular a permanent magnet-excited motor with a rotating rotor.
2. Description of the Prior Art
Electrical machines that function as motors or generators include subgroups whose rotors have permanent magnets that constitute poles with alternating polarities along the circumference of the rotor and as a rule cooperate with electrical coils of a stator concentrically encompassing the rotor. The electrical coils of the stator generate a rotating field that carries the rotor along with it at a synchronous speed. Depending on the activation of the stator coils, these machines can also be used as so-called stepping motors, which can be exactly set to predetermined positions, for example in order to drive actuators. A subgroup of these electrical machines with permanent magnet excitation is known in the literature as a transverse flux machine, for example see Michael Bork “Designs of Transverse Flux Machines” [Bauformen von Transversalflussmaschinen], Diss. 82, RWTH Aachen; Shaker-Verlag Aachen, 1997; pp. 6 ff. In the dissertation cited, the permanent magnets of the rotor form two annular arrangements spaced apart from each other on the circumference of the rotor, which cooperate with U-shaped magnetic yokes in the stator of the machines.
In the various designs of rotors with permanent magnet excitation, the permanent magnets are, for example, glued or cast in a ring around the circumference of the rotor and are then magnetized.
This has the disadvantage of requiring a special magnetizing device for the magnetization. In addition, the transitions of the alternating polarities along the rotor surface are imprecise due to this kind of magnetization, which has a negative effect on the machine output. In addition, with magnetizing devices of this kind, the individual permanent magnets cannot be embodied with an arbitrarily narrow width so that with a predetermined number of poles, it is possible that larger rotor diameters will be required than would be necessary for the machine output. In other designs of the rotor in which prefabricated tube sections made of magnetic material with premagnetized poles are slid onto the rotor and affixed, there is also, due to the brittleness of the material, the danger of material fractures due to internal stress after the magnet rings are placed onto the rotor. When machines of this kind are used in regions relevant to safety, for example as servomotors of an electrical steering mechanism in motor vehicles, it can be necessary to cover the magnet ring with an additional protective sleeve made of nonmagnetic material in order to prevent a failure of the machine due to breakage of a magnet ring.
The object of the current invention is to improve the attachment of permanent magnets to the rotor in electrical machines of this kind so that the flaws mentioned above can be largely prevented.